In recent years, as image sensors are made increasingly high-performance and compact, digital appliances furnished with an image input capability, such as cellular phones and portable information terminals provided with an imaging optical device, have been becoming more and more popular. And there have been increasing demands for further compactness and higher performance in imaging lens systems incorporated in imaging optical devices. Imaging lens systems for such applications conventionally have a three-element or four-element construction; today there are also proposed imaging lens systems having a five-element constructions.
As further compactness is aimed at as mentioned above, the exit pupil position comes closer to the image sensor. This increases the angle of incidence of image-side peripheral rays with respect to the image sensor, and causes shading, diminishing the amount of light. As a means for solving the problem, Patent Document 1 listed below proposes an imaging lens system having a five-element construction in which an aspherically shaped lens element having an inflection point elsewhere than at the intersection with the optical axis is used to achieve telecentricity in image-side rays. Imaging lens systems having a four-element construction are seen, for example, in Patent Documents 2 and 3 listed below.